Numerous items such as smart phones, smart watches, tablets, automobiles, aerial drones, appliances, aircraft, exercise aids, and game controllers may utilize motion sensors during their operation. In many applications, various types of motion sensors such as accelerometers and gyroscopes may be analyzed independently or together in order to determine varied information for particular applications. For example, gyroscopes and accelerometers may be used in gaming applications (e.g., smart phones or game controllers) to capture complex movements by a user, drones and other aircraft may determine orientation based on gyroscope measurements (e.g., roll, pitch, and yaw), and vehicles may utilize measurements for determining direction (e.g., for dead reckoning) and safety (e.g., to recognizing skid or roll-over conditions).
Motion sensors such as accelerometers and gyroscopes may be manufactured as microelectromechanical (MEMS) sensors that are fabricated using semiconductor manufacturing techniques. A MEMS sensor may include movable proof masses that can respond to forces such as linear acceleration (e.g., for MEMS accelerometers), angular velocity (e.g., for MEMS gyroscopes), pressure, and magnetic field. The operation of these forces on the movable proof masses may be measured based on the movement of the proof masses in response to the forces. In some implementations, this movement is measured based on distance between the movable proof masses and fixed electrodes, which form capacitors for sensing the movement.
The MEMS sensor may include multiple layers that are manufactured to collectively form the sensor cavity and components, such as a MEMS layer, cap layer, and substrate layer. These layers typically have parallel surfaces with respect to each other. The movable proof masses may be located in the MEMS layer. When fixed electrodes are also located within the MEMS layer, movement of the movable poof masses within the MEMS layer (in-plane movement) relative to the fixed electrodes may be used to measure a force along a plane of the MEMS layer. When fixed electrodes are located on another layer, movement of the movable poof masses outside the plane of the MEMS layer (out-of-plane movement) relative to the fixed electrodes may be used to measure a force out of the plane of the MEMS layer (e.g., perpendicular to the MEMS layer). The MEMS sensors may be designed based on expected relative locations of the movable proof masses and the fixed electrodes, both in the absence of external forces and in response to external forces. If a particular MEMS sensor departs from those expected relative locations due to factors such as manufacturing tolerances or wear, the measurement of the desired force by the sensor may be inaccurate.